The fabrication of fiberboard by the wet process (Thomas M. Maloney. Modern Particleboard and Dryprocess Fiberboard Manufacturing. Pullman, Wash., 1977) is known to make use of vegatable raw materials, viz. ground and suitably treated softwood, or hardwood, or mixtures thereof.
However this known method of fiberboard manufacture has an essential drawback in that it requires consumption in large quantities of wood presently available at high cost.
There is another known method for fiberboard manufacture, in which the consumption of high-cost wood is reduced by replacing it in part with additives, such as sawdust or bark (S. P. Rebrin. Tekhnologiya drevesnovoloknistykh plit (Fiberboard Technology). Lesnaya Promyshlennost Publishers, Moscow, 1982, pp. 5, 18, 19).
The use of wood processing or sawmilling wastes for the purpose of fiberboard manufacture by the wet process does make for a reduction in the consumption of expensive wood.
Yet the method in question is fraught with several difficulties. Thus, for example, the use of sawdust as an additive in fiberboard fabrication necessitates the use of specially designed grinding equipment, this raising the costs of and complicating the wet-process technology of fiberboard fabrication. Where bark is utilized as an additive, the quality of the fiberboard suffers, the detrimental effect being due to the bark moisture content normally not exceeding 20 percent. The relatively low moisture content of bark has a negative effect upon the quality of the fiber, with the fiberboard surface becoming non-uniform as a consequence. It is for this reason that the use of bark as a partial substitute for wood in fiberboard manufacture has but a limited scope of application.
Other partial substitutes for wood that can be used in fiberboard fabrication are sugar cane stems (USSR Pat. No. 211,452 dated 25.03.1966), hydrolysis lignin (USSR Inventor's Certificate No. 331,930 dated 25.12.1969), and activated sludge (USSR Inventor's Certificate No. 537,843 dated 23.05.1975).
The process that has the most widespread usage in fiberboard manufacture is the wet process using woodships as the principal raw material (S. P. Rebrin. Tekhnologiya drevesnovoloknistykh plit (Fiberboard Technology). Lesnaya Promyshlennost Publishers, Moscow, pp. 114-120).
This raw material is subjected to steam treatment in steaming units to be subsequently ground in mills. The wood pulp obtained as a result of woodship milling is additionally diluted with water to a concentration not exceeding 4 percent. Then the pulp is further ground and further diluted with water. The concentration of wood pulp in the water slurry thus obtained does not exceed 0.9 to 1.8 percent. In the sizing box, water-repellent agents are added to the water slurry, such as paraffin, oleic acid, sulphate soap, or a ceresin compound. The concentration of the water-repellent additive does not exceed 1 percent. The additive is introduced in the form of a finely dispersed emulsion. The emulsion envelopes wood fibers and fills the pores in the final product, preventing the ingress of moisture into the finished fiberboard. To assure a greater mechanical strength in fiberboard in the process of fabrication, binding agents are also added into the sizing box, such as, for example, phenol-formaldehyde resin, along with a suitable hardener, such as sulphuric acid. After mixing the aforesaid components in the sizing box, the resultant water slurry is cast over a wire screen, a fibermat forming upon the wire screen as a consequence. The mat is obtained in the following sequence of operations: pouring water slurry over the screen--filtering off water by gravity through the wire screen--removing water by suction with the aid of a vacuum unit and further by mechanical squeezing. As a result, a mat is obtained with a relative moisture content of about 80 percent. This is then forwarded to pressing rolls for edge trimming and further dewatering, with the relative moisture content being reduced here to 60 percent. This done, the fibermat is compacted at a temperature of 200.degree. to 215.degree. C. and a pressure of 5 to 5.5 MPa. In the compaction process, water is further removed so that the resulting fiberboard has a moisture content of 0.5 to 1.5 percent. To assure a high level of binder curing and thereby high strength characteristics, the compacted fiberboard is maintained in heat treatment ovens at a temperature of 160.degree. to 170.degree. C. for a period of four hours. Then the fiberboard is transferred to special chambers to be treated there with air having a relative humidity of 98 percent at a temperature of 65.degree. C. The fiberboard thus acquires an uniform moisture content throughout and shows no tendency to surface warping while in storage.
For all that the method as described above also has a number of essential drawbacks. Apart from the large amount of high-cost wood to be used in the wet-process fabrication of fiberboard, there are also waste waters to cause environmental pollution, since these contain such highly toxic substances as phenol and formaldehyde which find their way into waste waters from the phenol-formaldehyde resin binder. Another disadvantage is the length of time consumed in maintaining fiberboard in heat treatment chambers where they acquire mechanical strength due to the curing of the binder at high temperature. The large period of time required for binder curing is due to the low concentration in the product of sulphuric acid used as cure catalyst, while the low catalyst concentration is due to losses caused by dissolving in and entrainment with waste waters. Incomplete curing of the resin imparts toxicity to the finished product. This toxicity is due to the presence of uncured phenol and formaldehyde which will evaporate from the product into the surrounding air. Emission of said components and their presence in the surrounding air will cause various diseases of allergic nature in human beings.